The present invention relates to a method in the forming gap in a twin-wire web former of a paper machine, which gap is defined between two opposite forming wires. A pulp suspension jet is fed out of the discharge opening of the headbox of the paper machine into the forming gap. The pulp suspension jet has a certain free flight distance before it meets the forming wires.
The present invention also relates to a forming-gap arrangement in a twin-wire former of a paper machine, which arrangement includes a forming gap, into which a pulp suspension jet is fed out of a discharge opening of a headbox. The discharge opening is profiled in the transverse direction by means of a profile bar and a series of adjusting spindles acting upon the profile bar, and/or by equivalent means for adjusting the profile of the pulp suspension jet. The forming gap is defined between a first wire guided by the forming roll and a second wire guided by a breast roll situated opposite to the forming roll.
In prior art gap formers in paper machines, the pulp suspension jet is fed into a gap which is placed between the forming wires, a so-called forming gap, and becomes narrower, e.g., wedge-shaped in the flow direction of the suspension jet. In several gap formers, such as the formers marketed by the assignee under the trade mark "Speed Former HS".TM., the pulp suspension jet is directed towards an unsupported outer wire at a certain angle of impingement. The live pulp suspension jet produces instability in the unsupported wire, and, even more so, transverse wrinkles, wave formation and streaks at the edges of the web. This tendency to cause wrinkles and form waves produces variation in grammage both in the machine direction and in the transverse direction in the finished paper or board produced by the paper machine.
In gap forming techniques, the flight distance of the pulp suspension jet departing from the headbox is considered a critical factor in several respects. The relatively long flight distance of the jet subjects the jet to the effects of air flows in the gap, whereby the point of impingement of the jet may change and/or the face of the jet may be disintegrated. This results in a deterioration of the formation, and possibly also other properties, of the paper. In the absence of turbulence arising from differences in velocity produced by walls, the long flight distance of the jet also increases the re-flocculation of the fibers and causes a detrimental extent. Flocculation being the aggregation and coalescence of the pulp fibers into small lumps or loose clusters.
In typical prior art gap formers, two opposite forming rolls are utilized and are arranged inside the two wire loops to operate as breast rolls thereby forming a gap. Alternatively, one forming roll is used in the gap area and is placed inside the loop of the inner wire. The outer wire is passed into contact with the roll by means of a reversing roll or breast roll. Owing to the large-diameter breast rolls and forming rolls and owing to the guide rolls, the geometry of the forming gap usually becomes such that it is difficult to place the discharge opening of the headbox deep enough inside the forming gap, e.g., because the regulation means of the profile bar at the discharge opening require a considerably large amount of space.
A prior art solution to this problem is to shorten the flight distance of the discharge jet of the headbox by means of various stationary "turning bar" constructions, whereby the wire can be made to pass closer to the starting point of the jet. An example of this type of prior art construction is the forming-gap arrangement described in the assignee's Finnish Patent No. 86,752, in which, besides a "turning bar" an oblong stabilization bar is arranged inside the loop of the outer wire in the area of the bottom of the forming gap. The oblong stabilization bar stabilizes the run of the outer wire in the area of the bottom of the forming gap and removes water through the outer wire.
In conventional headboxes for gap formers, there is a precisely machined profile bar for the ultimate smoothing of the pulp jet and for the control of the grammage profile. The thickness of the pulp flow, i.e., the grammage profile of the paper web, is controlled, in a manner known in prior art, by bending the profile bar by means of spindles. As stated above, these regulation means, i.e., spindles, are large devices and prevent positioning of the lips of the headbox as deep as desired in the forming gap itself. Thus, in the prior art gap formers, the free jet length is limited in a range from about 200 mm to about 300 mm, depending on the width of the machine and on the diameter of the forming and breast rolls.
In view of solving these problems, in the assignee's Finnish Patent No. 84,735, a method is described in which the thickness profile of the pulp suspension jet in the transverse direction is regulated by profiling the mantle of at least one breast roll arranged inside the forming wire in the area of the forming gap. The forming-gap arrangement of FI 84,735 comprises at least one adjustable-crown breast roll and a series of hydraulic loading members arranged in the interior of the breast roll in the area of the forming gap. The hydraulic loading members are arranged to be loaded by hydraulic pressure. The adjustable-crown breast roll is arranged to shape the forming gap in a manner such that, by regulating the deflection of its mantle, the transverse thickness profile of the pulp suspension jet fed into the forming gap can be controlled.
In the prior art forming-gap arrangements of twin-wire formers, generally a so-called "roll supported at the middle" (hereinafter referred to as a mid-support roll) is used as the reversing roll or breast roll placed opposite to the former roll. In this position, the mid-support rolls create a number of different problems, of which the unfavorably large length of the discharge jet arising from the large diameter of the mid-support roll, was already discussed above. Second, the form of deflection of the mid-support roll is unfavorable. For this reason, the run of the wire guided by the mid-support roll is unstable and poorly controllable, especially across a width of about 1 meters to about 2 meters at the edges of the wire. This results from the relative difference between the deflection lines of the outer mantle of the mid-support roll and of the other rolls in the wire circulation.
The use of the mid-support roll as a breast roll restricts the setting of the tension of the forming wire guided by the mid-support roll. The tension of the wire and the variation of the tension in the transverse direction affects the draining of water in the twin-wire zone. At present, the operation takes place in a wire-tension range of from about 5 kN/m to about 8 kN/m, but a need has arisen to raise the level of wire tension. It is estimated that in the future it will be necessary to employ a tension range of about 10 kN/m to about 12 kN/m. One object of the present invention is also to permit web former operations with such increased wire-tension.
The problems discussed above have also been discussed in Finnish Patent Application No. FI 920500 (corresponding to German Patent Application No. DE 41 05 215.3 and Canadian Patent Application No. 2,061,517) to Messrs. J. M. Voith GmbH. In FI 920500, a device is described in which one or both of the breast rolls of the twin-wire former are supported by means of a hydrostatic pressure bearing arranged externally in relation to the roll. This arrangement is, however, unfavorable both in view of the construction and in view of the operation, among other things, because the upper breast roll requires specific support means. Another disadvantage arises because the hydrostatic pressure bearing is placed on an upper sector of the roll, in which case the weight of the roll is supported by wire tensioning forces only.